Gate driver circuit for reducing deadtime inefficiencies

ABSTRACT

A driver circuit includes three sub-circuits. A first sub-circuit is configured to generate a drive current output by the driver circuit through an output node during first and second regions of operation and includes: a diode coupled to the output node and a first transistor, and a second transistor coupled to the first transistor and a current mirror. A second sub-circuit is configured to generate the drive current during the first and second and a third region of operation and includes: a third transistor coupled to the output node; and a fourth transistor. A third sub-circuit is configured to generate the drive current during the third region of operation and includes: a current source coupled to the current mirror and a buffer; and a fifth transistor coupled to the third transistor and the fourth transistor and configured to receive an output of the buffer.

CROSS REFERENCE TO RELATED APPLICATION(S)

This application is a divisional of U.S. application Ser. No. 16/834,362filed Mar. 30, 2020, which is a divisional of U.S. application Ser. No.16/406,104, filed May 8, 2019, now U.S. Pat. No. 10,642,306, both ofwhich are incorporated herein by reference in its entirety.

BACKGROUND

H bridge drivers include two high side transistors and two low sidetransistors, and are configured such that each high side transistor isin series with a respective low side transistor and a load is coupled tothe nodes between the pairs of high side and low side transistors. Eachpair of high side and low side transistors is called a half bridge. Agate driver circuit converts a control signal to a power signal that canefficiently turn on and off each transistor in the H bridge driver. Toprevent shoot-through conditions within a half bridge during transitionsfrom high to low or low to high, dead time is inserted between turningoff of one transistor and turning on of the other. The length of thedead time is often equal to the time required to turn on or turn off atransistor in the half bridge driver. However, dead time limits theoperating frequency of the H bridge driver and allows power dissipationthrough transistors in the H bridge driver.

Some gate driver circuits reduce power dissipation during dead time byturning the transistor on and off more quickly. This may increase theefficiency of the H bridge driver, but at the cost of creatingelectromagnetic interference (EMI) in the integrated circuit (IC),disrupting operation of other circuits in the IC. Some gate drivercircuits balance the need for quick turn on and turn off times withcontrolled EMI by implementing comparators to detect different regionsof operation for each transistor during turn on and turn off, andquickening only the regions that are unlikely to cause EMI. However,these comparators are often area expensive, especially for a high-sidegate driver requiring a floating power supply.

SUMMARY

In some implementations, a driver circuit adjusts a drive current duringtransistor turn on and turn off based on the transistor's region ofoperation. The driver circuit includes a first sub-circuit, a secondsub-circuit, and a third sub-circuit. The first sub-circuit isconfigured to generate a drive current output by the driver circuitthrough an output node during a first region of operation and a secondregion of operation and includes: a diode coupled to the output node anda first transistor, and a second transistor coupled to the firsttransistor and a current mirror. The second sub-circuit is configured togenerate the drive current during the first and the second regions ofoperation and a third region of operation and includes: a thirdtransistor coupled to the output node; and a fourth transistor. Thethird sub-circuit is configured to generate the drive current during thethird region of operation and includes: a current source coupled to thecurrent mirror and a buffer; and a fifth transistor coupled to the thirdtransistor and the fourth transistor and configured to receive an outputof the buffer.

BRIEF DESCRIPTION OF THE DRAWINGS

For a detailed description of various examples, reference will now bemade to the accompanying drawings in which:

FIG. 1 illustrates an example H bridge motor driver.

FIG. 2 shows the gate to source voltages of a high side transistor and alow side transistor in an example H bridge motor driver over time.

FIGS. 3A-B show the gate to source voltages of a high side transistorand a low side transistor, and the voltages on the output nodes of anexample H bridge motor driver over time.

FIG. 4 illustrates an example pull-up gate driver circuit for atransistor in the example H bridge motor driver of FIG. 1.

FIG. 5 illustrates an example pull-down gate driver circuit for atransistor in the example H bridge motor driver of FIG. 1.

FIG. 6 illustrates an example gate driver circuit for a transistor inthe example H bridge motor driver of FIG. 1.

FIG. 7 shows the gate to source voltage of a transistor in an example Hbridge motor driver, the impedance seen by an example gate drivercircuit due to a charge pump protection circuit, the current input toand output by a current mirror within the example gate driver circuit,and a mono-shot signal from the charge pump protection circuit overtime.

DETAILED DESCRIPTION

As noted above, some gate driver circuits cause electromagneticinterference (EMI) in a circuit (e.g., an integrated circuit (IC)),occupy a large area on an IC, or cause power dissipation in acorresponding H bridge driver circuit. The disclosed example gate drivercircuits detect the different regions of operation for each transistorin the corresponding H bridge driver circuit during turn on and turnoff, and quicken only the regions that are unlikely to cause EMI,reducing power dissipation in the H bridge driver circuit. The disclosedexamples use particular portions of the gate driver circuit duringmultiple regions of operation and include only two high voltage, areaexpensive transistors, reducing the total area occupied by the gatedriver circuit. The two high voltage transistors also protect thecorresponding H bridge driver circuit in the event of a short circuit byautomatically disconnecting the H bridge driver circuit from the chargepump or boost converter supplying the increased supply voltage to thegate terminals of transistors in the H bridge driver circuit.

A disclosed example gate driver circuit includes a first bufferconfigured to receive a control signal for a particular transistor inthe associated H bridge driver circuit. A first transistor is coupled tothe output of the first buffer and to an output node of the gate drivercircuit and provides a drive current to the gate terminal of theparticular transistor in the associated H bridge driver circuit during afirst region of operation, while the gate to source voltage of theparticular transistor increases to the particular transistor's thresholdvoltage during turn on. Once the gate to source voltage of theparticular transistor increases to the threshold voltage and plateauswhile the drain to source voltage of the particular transistordecreases, the first transistor turns off and provides no drive currentto the gate terminal of the particular transistor.

A second transistor is coupled to a first current mirror and the outputnode, and configured to receive the control signal for the particulartransistor in the associated H bridge driver circuit. During a secondregion of operation, while the gate to source voltage of the particulartransistor plateaus and the drain to source voltage of the particulartransistor decreases during turn on, the second transistor and the firstcurrent mirror provide a carefully controlled drive current to the gateterminal of the particular transistor. The drive current is chosen tochange the voltage on an output node in the associated H bridge drivercircuit at a particular rate, such that the voltage change generates EMIbelow a particular threshold associated with the IC and reduces noiseprovided to other circuits on the IC.

A third transistor is coupled to the output node and an inverter, andconfigured to receive the control signal for the particular transistorin the associated H bridge driver circuit. A fourth transistor iscoupled to the output node and a supply voltage node, and configured toreceive the output of the inverter. When the voltage on the output nodeis greater than a voltage of the control signal, the third and fourthtransistors turn on, and additional drive current flows from the supplyvoltage node through the fourth transistor to the output node of thegate driver circuit and the gate terminal of the particular transistorin the associated H bridge driver circuit, completing turn on.

A fifth transistor is coupled to the third transistor and a sixthtransistor, and configured to receive the control signal for theparticular transistor in the associated H bridge driver circuit. Duringa fourth region of operation and when the control signal indicated theparticular transistor should be turned off, pull-down current flows fromthe output node through the third and fifth transistor to the sixthtransistor and a second current mirror. The sixth transistor isconfigured to receive the inverse of the control signal for theparticular transistor. As the voltage on the output node approaches avoltage of the control signal during a fifth region of operation, thepull-down current through the third, fifth, and sixth transistorsdecreases until it stops when the voltage on the output node equals thevoltage of the control signal and the threshold voltages of the thirdand fifth transistors, which turn off.

As the pull-down current through the third, fifth and sixth transistorsdecreases and stops, additional pull-down current flows through thefirst transistor, the first buffer, and a seventh transistor coupled tothe first buffer, carefully controlled to change the voltage on theoutput node in the associated H bridge driver circuit at a particularrate and to generate EMI below the particular threshold associated withthe IC during the fifth region of operation. When the pull-down currentthrough the third, fifth and sixth transistors stops, a current sourcecoupled to the second current mirror provides current to an input of asecond buffer. The output of the second buffer is coupled to a gateterminal of an eighth transistor, which is coupled to the seventhtransistor and the first buffer. When the current source providescurrent to the second buffer during a sixth region of operation, theeighth transistor turns on and increases the pull-down current throughthe first transistor and the first buffer, completing turn off of theparticular transistor in the associated H bridge driver circuit.

FIG. 1 illustrates an example H bridge motor driver 100. Although FIG. 1illustrates an H bridge implemented as a motor driver, H bridges and thegate driver circuits described herein can be used in a variety ofapplications, including other kinds of power electronics and the like.Example H bridge motor driver 100 includes high-side power field-effecttransistors (FETs) M_H1 and M_H3 and low-side power FETs M_H2 and M_H4.A source terminal of M_H1 is coupled to a drain terminal of M_H2 at node115, forming a half-bridge configuration. A source terminal of M_H3 iscoupled to a drain terminal of M_H4 at node 135, forming anotherhalf-bridge configuration. The drain terminals of M_H1 and M_H3 arecoupled to receive a supply voltage Vmotor at node 105. The sourceterminals of M_H2 and M_H4 are coupled to receive a common mode voltageat node 160. In some examples, the common mode voltage at node 160 isground. Node 115 and node 135 form output nodes of the H bridge motordriver 100 and load 150 is coupled to nodes 115 and 135.

The gate terminals of each of M_H1, M_H2, M_H3, and M_H4 are coupled torespective gate driver circuits. The gate terminal of M_H1 is coupled toreceive a gate driving current from gate driver circuit 110. The gateterminal of M_H2 is coupled to receive a gate driving current from gatedriver circuit 120. The gate terminal of M_H3 is coupled to receive agate driving current from gate driver circuit 130. The gate terminal ofM_H4 is coupled to receive a gate driving current from gate drivercircuit 140.

H bridge motor driver 100 includes metal oxide semiconductorfield-effect transistors (MOSFETs). M_H1, M_H2, M_H3, and M_H4 aren-type MOSFETs (NMOS) in this example. In other examples, one or more ofM_H1, M_H2, M_H3, and M_H4 are p-type MOSFETs (PMOS) or bipolar junctiontransistors. A bipolar junction transistor includes a base correspondingto the gate terminal, and a collector and an emitter corresponding tothe drain and source terminals. The base of a bipolar junctiontransistor and the gate terminal of a MOSFET are also called controlinputs. The collector and emitter of a bipolar junction transistor andthe drain and source terminals of a MOSFET are also called currentterminals.

FIG. 2 shows the gate to source voltage Vgs_HS 210 of M_H1 and the gateto source voltage Vgs_LS 220 of M_H2 in example H bridge motor driver100 over time. To prevent shoot-through current, the switching schemecontrolling M_H1 and M_H2 includes a dead-time 250 in which neither thehigh side transistor M_H1 nor the low side transistor M_H2 are on. Thisensures that M_H1 is fully off before M_H2 turns on. As discussedpreviously, dead-time 250 slows the switching frequency achievable by Hbridge motor driver 100 and increases power dissipated through H bridgemotor driver 100. To counter this, some motor drivers increase the slewrate, the speed at which the transistors turn on and off, reducing thepropagation delay. Increasing the slew rate may also increase EMI andcreate noise that interferes with operation of other circuits within theIC. As EMI impacts operation of other circuits, some gate drivercircuits prioritize the slew rate and EMI requirements of the IC overswitching frequency.

FIG. 3A shows the gate to source voltage Vgs_HS 360 and the voltage onnode 115 Vnode115 370 of the high side transistor M_H1 in the example Hbridge motor driver 100 over time. FIG. 3A is described in the contextof M_H1 in H bridge motor driver 100, but high side transistor M_H3 orother high side transistors in other H bridge drivers will experiencesimilar gate to source voltages and voltages on an output node. Inregions 305-315, the transistor is turned on. In region 320, M_H1 iskept in an on state while in regions 325-335, the transistor is turnedoff. Region 305 is the transistor turn-on region, in which the gate tosource capacitance Cgs of M_H1 is charged to the threshold voltage Vth.When Vgs_HS 360 is greater than Vth, the transistor conducts current.The ramp rate in region 305 is proportional to

$\frac{Idrive}{\left( {{Cgs} + {Cgd}} \right)},$

where Idrive is the current applied to the gate terminal of thetransistor and Cgd is the gate to drain capacitance of the transistor.

Region 310 is known as the Miller region, in which Vgs_HS 360 remainsconstant and Vnode115 370 increases as Cgd is charged. The length oftime spent in region 310, tSlew, is proportional to

$\frac{Idrive}{Cgd}.$

Because region 310 and the increasing Vnode115 370 can create EMI andnoise in other circuits on the IC, tSlew and the corresponding drivecurrent Idrive are determined according to the requirements of the ICand the amount of EMI other circuits on the IC can withstand. Once Cgdis charged, the conducting channel of M_H1 is fully enhanced by applyinga higher gate drive voltage and drive current Idrive to the gateterminal in region 315, known as the RDSON enhancement region. The ramprate in region 315 is proportional to

$\frac{Idrive}{\left( {{Cgs} + {Cgd}} \right)}.$

Once Vgs_HS 360 reaches the on voltage of the transistor and entersregion 320, M_H1 is in an on state.

When the transistor is turned off, Vgs_HS 360 decreases as the gatecapacitance of the M_H1, Cgs+Cgd, is discharged to the Miller plateaulevel in region 325. The gate current is supplied by the gatecapacitance and the ramp rate is proportional to

$\frac{Idrive}{{Cgs} + {Cgd}}.$

Once the gate voltage reaches the Miller plateau level, Vnode115 370decreases as Cgd discharges further. The length of time spent in region330, tSlew, is proportional to

$\frac{Idrive}{Cgd}.$

As in region 310, region 330 and the decreasing Vnode115 370 can causeEMI and noise in other circuits on the IC. TSlew and the correspondingdrive current are determined according to the requirements of the IC andthe amount of EMI other circuits on the IC can withstand. In region 335,both Cgs and Cgd are discharged to zero volts, decreasing Vgs_HS 360 tozero volts and turning off the transistor. The ramp rate is proportionalto

$\frac{Idrive}{\left( {{Cgs} + {Cgd}} \right)}.$

To increase switching efficiency and optimize turn on, turn off, and thethree regions of operation in each while maintaining the EMI causedduring switching within the requirements of the IC, the drive currentIdrive is varied in each region of operation. The varying drive currentIdrive speeds transistor turn on in regions 305 and 315 and transistorturn off in regions 325 and 335 but carefully controls turn on and turnoff speed in regions 310 and 330, such that the EMI created stays withinacceptable limits for the IC.

FIG. 3B shows the gate to source voltage Vgs_LS 380 and the voltage onnode 135 Vnode135 390 of the low side transistor M_H4 in the example Hbridge motor driver 100 over time. FIG. 3B is described in the contextof M_H4 in H bridge motor driver 100, but low side transistor M_H2 orother low side transistors in other H bridge drivers will experiencesimilar gate to source voltages and voltages on an output node. Just asin FIG. 3A, regions 305-315 correspond to transistor turn on. In region320, M_H4 is kept in an on state while in regions 325-335, thetransistor is turned off. Region 305 is the transistor turn-on region,in which the gate to source capacitance Cgs of M_H4 is charged to thethreshold voltage Vth. When Vgs_LS 380 is greater than Vth, thetransistor conducts current. The ramp rate in region 305 is proportionalto

$\frac{Idrive}{\left( {{Cgs} + {Cgd}} \right)},$

where Idrive is the current applied to the gate terminal of thetransistor and Cgd is the gate to drain capacitance of the transistor.

Region 310 is known as the Miller region, in which Vgs_LS 380 remainsconstant and Vnode135 390 decreases as Cgd is charged. The length oftime spent in region 310, tSlew, is proportional to

$\frac{Idrive}{Cgd}.$

Because region 310 and the decreasing Vnode135 390 can create EMI andnoise in other circuits on the IC, tSlew and the corresponding drivecurrent Idrive are determined according to the requirements of the ICand the amount of EMI other circuits on the IC can withstand. Once Cgdis charged, the conducting channel of M_H4 is fully enhanced by applyinga higher gate drive voltage and drive current Idrive to the gateterminal in region 315, known as the RDSON enhancement region. The ramprate in region 315 is proportional to

$\frac{Idrive}{\left( {{Cgs} + {Cgd}} \right)}.$

Once Vgs_LS 380 reaches the on voltage of the transistor and entersregion 320, M_H4 is in an on state.

When the transistor is turned off, Vgs_LS 380 decreases as the gatecapacitance of the M_H4, Cgs+Cgd, is discharged to the Miller plateaulevel in region 325. The gate current is supplied by the gatecapacitance and the ramp rate is proportional to

$\frac{Idrive}{{Cgs} + {Cgd}}.$

Once the gate voltage reaches the Miller plateau level, Vnode135 390increases as Cgd discharges further. The length of time spent in region330, tSlew, is proportional to

$\frac{Idrive}{Cgd}.$

As in region 310, region 330 and the increasing Vnode135 390 can causeEMI and noise in other circuits on the IC. TSlew and the correspondingdrive current are determined according to the requirements of the IC andthe amount of EMI other circuits on the IC can withstand. In region 335,both Cgs and Cgd are discharged to zero volts, decreasing Vgs_LS 380 tozero volts and turning off the transistor. The ramp rate is proportionalto

$\frac{Idrive}{\left( {{Cgs} + {Cgd}} \right)}.$

To increase switching efficiency and optimize turn on, turn off, and thethree regions of operation in each while maintaining the EMI causedduring switching within the requirements of the IC, the drive currentIdrive is varied in each region of operation just as in FIG. 3A fortransistor M_H1. The varying drive current Idrive speeds transistor turnon in regions 305 and 315 and transistor turn off in regions 325 and 335but carefully controls turn on and turn off speed in regions 310 and330, such that the EMI created stays within acceptable limits for theIC.

FIG. 4 illustrates an example pull-up gate driver circuit 400 for atransistor in the example H bridge motor driver 100 of FIG. 1. Pull-upgate driver circuit 400 is shown in connection with M_H1 and gate drivercircuit 110, but may be used in gate driver circuits 120, 130, and 140as well. Pull-up gate driver circuit 400 includes NMOS transistors M1and M2 and PMOS transistors M_A, M3, M4, M5, M6, and M7, as well asinverters 415 and 440. In other examples, one or more of M1 and M2 arePMOS transistors and one or more of M_A, M3, M4, M5, M6, and M7 are NMOStransistors. In other examples, one or more of M1, M2, M_A, M3, M4, M5,M6, and M7 are bipolarjunction transistors.

Inverter 415 is configured to receive a control signal VIN 435 andcoupled to the gate terminal of M_A. The source terminal of M_A iscoupled to receive a supply voltage Vsupply at node 450 and the drainterminal of M_A is coupled to the source terminal of M2. In someexamples, Vsupply is equal to Vmotor and to 5V. The gate terminal of M2is configured to receive a biasing voltage equal to two times thethreshold voltage Vth of M_H1 in this example. The drain terminal of M2is coupled to output node 495 of pull-up gate driver circuit 400. WhenVIN 435 indicates M_H1 should be turned on, a logic low signal isapplied to the gate terminal of M_A, causing M_A to turn on. M_A actslike a closed switch and allows a drive current Idrive to flow from thesupply voltage on node 450 through M2 to output node 495 and byextension the gate terminal of M_H1 during region 305. M2 enters thecutoff region when the voltage on output node 495 is approximately Vth,acting like an open switch and disconnecting M_A and current from node450 from output node 495. This transitions Vgs_HS 360 of M_H1 fromregion 305 to region 310.

The gate terminal of M1 is configured to receive VIN 435. The drainterminal of M1 is connected to the drain and gate terminals of M3 andthe gate terminal of M4 at node 425. The source terminal of M1 isconnected to charge pump protection circuit 460, which is configured toprevent a charge pump or boost converter coupled to node 405 fromcollapsing in the event of a short circuit at node 115. The sourceterminals of M3 and M4 are coupled to node 405 and configured to receivea charge pump or boost converter voltage Vboost. M3 and M4 function ascurrent mirror 430. The drain terminal of M4 is coupled to the sourceterminal of M5. The gate terminal of M5 is configured to receive Vmotorand the drain terminal of M5 is coupled to output node 495. Duringregion 310, current mirror 430 and M5 provide the drive current Idrivefrom the charge pump or boost converter to output node 495.

The gate terminal of M6 is configured to receive Vmotor and the drainterminal is coupled to output node 495. The source terminal is coupledto the input of inverter 440. Inverter 440 is coupled to the gateterminal of M7. The source terminal of M7 is coupled to node 405 andconfigured to receive a charge pump or boost converter voltage Vboost.The drain terminal of M7 is coupled to output node 495. During region310, M6 compares the voltage on output node 495, its drain terminal, toVmotor, applied to its gate terminal. When the voltage on output node495 exceeds Vmotor, M6 turns on and the source terminal of M6 has thesame voltage as the voltage on output node 495, causing the input toinverter 440 to be logic high. Inverter 440 converts the logic highsignal to a logic low signal and applies it to the gate terminal of M7,causing M7 to turn on and act as a closed switch. This ends region 310and transitions Vgs_HS 360 into region 315. M7 allows additional currentto flow from Vboost on node 405 to output node 495, increasing the drivecurrent Idrive applied to the gate terminal of M_H1. When Vgs_HS 360reaches the on voltage of M_H1, pull-up of M_H1 is complete and M_H1 ison.

FIG. 5 illustrates an example pull-down gate driver circuit 500 for atransistor in the example H bridge motor driver 100 of FIG. 1. Pull-downgate driver circuit 500 is shown in connection with M_H1 and gate drivercircuit 110, but may be used in gate driver circuits 120, 130, and 140as well. Pull-down gate driver circuit 500 includes NMOS transistors M2,M9, M10, M11, M12, and M13 and PMOS transistors M_B, M_C, and M8, aswell as diode 505, current source 535, and buffer 540. In otherexamples, one or more of M2, M9, M10, M11, M12, and M13 are PMOStransistors and one or more of M_B, M_C, and M8 are NMOS transistors. Inother examples, one or more of M2, M_B, M_C, M8, M9, M10, M11, M12, andM13 are bipolarjunction transistors.

M_C is coupled to node 405 and output node 495, and configured toreceive a voltage biasing it in the off state and acting as an openswitch, disconnecting Vboost on node 450 from output node 495. Diode 505is coupled to output node 495 and the source terminal of M8. The gateterminal of M8 is configured to receive Vmotor and the drain terminal ofM8 is coupled to the drain terminals of M_B and M9 at node 510. The gateterminal of M_B is configured to receive the voltage on node 115 and thesource terminal of M_B is coupled to output node 495. The gate terminalof M9 is configured to receive VIN 435 and the source terminal of M9 iscoupled to the gate terminals of M10 and M11 and the drain terminal ofM10 at node 520. The source terminals of M10 and M11 are coupled tocommon mode node 455. The drain terminal of M11 is coupled to the outputof current source 535 and the input of buffer 540 at node 530. M10 andM11 function as current mirror 525. The output of buffer 540 is coupledto the gate terminal of M12.

The drain terminal of M2 is connected to the drain terminal of M_C atoutput node 495 and the source terminal of M2 is connected to the drainterminals of M12 and M13 at node 515. The gate terminal of M2 isconfigured to receive a biasing voltage to keep it on and acting as aclosed switch, in this example two times the threshold voltage of M_H1.The source terminals of M12 and M13 are coupled to common mode node 455.The gate terminal of M13 is biased as current source by the charge pumpprotection circuit 460. The gate terminal of M12 is coupled to theoutput of buffer 540. During turn off, the gate terminal of M13 receivesa signal biasing it to keep it on and acting as a closed switch. Thiscreates a path for Ipulldown current through M2 and M13 to common modenode 455.

When VIN 435 indicates M_H1 should be turned off, Ipulldown currentflows through M2 and M13 to common mode node 455 and Istrong currentflows through diode 505, M8, M9, and M10 to common mode node 455,decreasing Vgs_HS 360 during region 325. While Istrong current flowsthrough M10 in current mirror 525, M11 acts as a closed switch andcauses the current from current source 535 to flow through M11 to commonmode node 455. This causes the voltage on node 530 input to buffer 540to be a logic low signal. This logic low signal is supplied to the gateterminal of M12, keeping it in an off state and acting as an openswitch.

As the voltage on output node 495 approaches Vmotor plus Vth plus thevoltage across diode 505, Istrong current through M8 approaches zero andIpulldown current remains steady. This provides a substantially constantpulldown current to output node 495 and the gate terminal of M_H1,allowing the voltage on node 115 to decrease at a rate appropriate forthe EMI requirements of the IC and causing Vgs_HS 360 to transition fromregion 325 to region 330. When Vgs_HS 360 decreases below Vmotor plusVth plus the voltage across diode 505, it has transitioned from region330 to region 335 and M8 is turned off, acting as an open switch suchthat no current flows through M9 to current mirror 525. This allows thevoltage on node 530 to increase and current source 535 to provide itscurrent to the input of buffer 540. This causes the input to and outputfrom buffer 540 to be a logic high signal. The logic high signal issupplied to the gate terminal of M12, causing it to turn on and act as aclosed switch. This creates another path for current to flow through,increasing Ipulldown current and speeding the decrease in Vgs_HS 360during region 335. When Vgs_HS 360 becomes zero volts, M_H1 is fullyturned off.

FIG. 6 illustrates an example gate driver circuit 600 for a transistorin the example H bridge motor driver 100 of FIG. 1, combining thefeatures of pull-up gate driver circuit 400 and pull-down gate drivercircuit 500. Gate driver 600 is shown in connection with M_H1 and gatedriver circuit 110, but may be used in gate driver circuits 120, 130,and 140 as well. Gate driver circuit 600 includes NMOS transistors M1,M2, M9, M10, M11, M12, and M13 and PMOS transistors M3, M4, M5, M6, M7,and M8, as well as inverter 440, current source 535, and buffers 415 and540. Only M6 and M7 are high voltage PMOS with high gate capacitance,occupying a relatively large area on the IC. The remaining transistorsare small, low power FETs. In other examples, one or more of M1, M2, M9,M10, M11, M12, and M13 are PMOS transistors and one or more of M3, M4,M5, M6, M7, and M8 are NMOS transistors. In other examples, one or moreof M1, M2, M3, M4, M5, M6, M7, M8, M9, M10, M11, M12, and M13 arebipolarjunction transistors.

Buffer 415 is configured to receive VIN 435 and is coupled to the sourceterminal of M2 and the drain terminal of M13. In this example, buffer415 includes complementary MOSFET inverters. An inverter 420 isconfigured to receive VIN 435 and is coupled to the gate terminals ofPMOS transistor M_Inv1 and NMOS transistor M_Inv2. The source terminalof M_Inv1 is coupled to a supply voltage Vsupply, which in some examplesis equal to Vmotor and to 5V. The drain terminal of M_Inv1 is coupled tothe drain terminal of M_Inv2. The source terminal of M_Inv2 is coupledto the drain terminal of M13 at node 515. The source terminal of M2 iscoupled to the drain terminals of M_Inv1 and M_Inv2. In gate drivercircuit 600, M_Inv1 acts as M_A in pull-up gate driver circuit 400 inFIG. 4.

The gate terminal of M2 is configured to receive a biasing voltage equalto two times the threshold voltage Vth of M_H1 in this example, and thedrain terminal of M2 is coupled to output node 495 of gate drivercircuit 600. When VIN 435 indicates M_H1 should be turned on, buffer 415allows a drive current Idrive to flow from the supply voltage throughM_Inv1 and M2 to output node 495 and by extension the gate terminal ofM_H1 during region 305. M2 turns off when the voltage on output node 495is approximately Vth, acting like an open switch and disconnectingbuffer 415 and current from its power supply from output node 495. Thistransitions Vgs_HS 360 of M_H1 from region 305 to region 310.

The gate terminal of M1 is configured to receive VIN 435. The drainterminal of M1 is connected to the drain and gate terminals of M3 andthe gate terminal of M4 at node 425. The source terminal of M1 isconnected to charge pump protection circuit 460, which is configured toprevent a charge pump or boost converter coupled to node 405 fromcollapsing in the event of a short circuit at node 115. The sourceterminals of M3 and M4 are coupled to node 405 and configured to receivea charge pump or boost converter voltage Vboost. M3 and M4 function ascurrent mirror 430. The drain terminal of M4 is coupled to the sourceterminal of M5. The gate terminal of M5 is configured to receive Vmotorand the drain terminal of M5 is coupled to output node 495. Duringregion 310, current mirror 430 and M5 provide the drive current Idriveto output node 495.

The gate terminal of M6 is configured to receive Vmotor and the drainterminal of M6 is coupled to output node 495. The source terminal iscoupled to the input of inverter 440 at node 605. Inverter 440 iscoupled to the gate terminal of M7. The source terminal of M7 is coupledto node 405 and configured to receive a charge pump or boost convertervoltage Vboost. The drain terminal of M7 is coupled to output node 495.During region 310, M6 compares the voltage on output node 495, its drainterminal, to Vmotor, applied to its gate terminal.

When the voltage on output node 495 exceeds Vmotor, M6 turns on and thecurrent input to inverter 440 from the source terminal of M6 goes logichigh. Inverter 440 converts the logic high signal to a logic low signaland applies the logic low signal to the gate terminal of M7, causing M7to turn on and act as a closed switch. This ends region 310 andtransitions Vgs_HS 360 into region 315. M7 allows additional current toflow from Vboost on node 405 to output node 495, increasing the drivecurrent Idrive applied to the gate terminal of M_H1 and speeding theincrease of Vgs_HS 360 during region 315. When Vgs_HS 360 reaches the onvoltage of M_H1, pull-up of M_H1 is complete and M_H1 is on.

M_H1 is kept on during region 320 by M7, which maintains Vboost onoutput node 495. In the event of a short circuit on node 115, thevoltage on output node 495 and the drain terminal of M6 will decreaserapidly, causing M6 to turn off and the input to inverter 440 to belogic low. Inverter 440 converts the logic low signal to a logic highsignal and applies it to the gate terminal of M7, causing M7 to turnoff. This causes M7 to act as an open switch and disconnects the chargepump from output node 495 and the short circuit. Any short circuitcurrent from charge pump before M7 disconnects it from output node 495is absorbed by a Zener diode 165 in H bridge motor driver circuit 100,which is connected to the gate and source terminals of M_H1. M_H2, M_H3,and M_H4 each have a Zener diode 165 coupled to their respective gateand source terminals.

The source terminal of M8 is coupled to the source terminal of M6 andthe input to inverter 440 at node 605. In gate driver circuit 600, M6acts as diode 505 in pull-down gate driver circuit 500. The gateterminal of M8 is configured to receive Vmotor and the drain terminal ofM8 is coupled to the drain terminal of M9 at node 510. The gate terminalof M9 is configured to receive VIN 435 and the source terminal of M9 iscoupled to the gate terminals of M10 and M11 and the drain terminal ofM10 at node 520. The source terminals of M10 and M11 are coupled tocommon mode node 455. The drain terminal of M11 is coupled to the outputof current source 535 and the input of buffer 540 at node 530. M10 andM11 function as current mirror 525.

The drain terminals of M12 and M13 are coupled to the source terminal ofM_Inv2 at node 515 and the source terminals of M12 and M13 are coupledto common mode node 455. The gate terminal of M13 is coupled to theshoot-through protection circuit 460. The gate terminal of M12 iscoupled to the output of buffer 540. During turn off, the gate terminalof M13 receives a signal biasing it on and acting as a closed switch.This creates a path for Ipulldown current from output node 495 to commonmode node 455.

When VIN 435 indicates M_H1 should be turned off, Ipulldown currentflows through M2, M_Inv2, and M13 to common mode node 455 and Istrongcurrent flows through M6, M8, M9, and M10 to common mode node 455,decreasing Vgs_HS 360 during region 325. In gate driver circuit 600,M_Inv2 acts as M_B in pull-down gate driver circuit 500 and M6 acts asdiode 505 in pull-down gate driver circuit 500. While Istrong currentflows through M10 in current mirror 525, M11 acts as a closed switch andcauses the current from current source 535 to flow through M11 to commonmode node 455. This causes the voltage on node 530 input to buffer 540to be a logic low signal. This logic low signal is supplied to the gateterminal of M12, keeping it in an off state and acting as an openswitch.

As the voltage on output node 495 approaches Vmotor plus Vth, Istrongcurrent through M8 approaches zero while Ipulldown current remainssteady. This provides a substantially constant drive current to outputnode 495 and the gate terminal of M_H1, allowing the voltage on node 115to decrease at a rate appropriate for the EMI requirements of the IC andcausing Vgs_HS 360 to transition from region 325 to region 330. WhenVgs_HS 360 decreases below Vmotor plus Vth, it has transitioned fromregion 330 to region 335 and M8 transitions into the cut off region,acting as an open switch such that Istrong current is zero and nocurrent flows through M9 to current mirror 525. This allows currentsource 535 to provide current to the input of buffer 540, causing theinput to and output from buffer 540 to be a logic high signal. The logichigh signal is supplied to the gate terminal of M12, causing it to turnon and act as a closed switch. This creates another path for current toflow through, increasing Ipulldown current and speeding the decrease inVgs_HS 360 during region 335. When Vgs_HS 360 becomes zero volts, M_H1is fully turned off.

Gate driver circuit 600 is area efficient, in that it includes only twolarge area, high voltage transistors M6 and M7. Reuse of M2 in pull-upregion 305 and pull-down regions 325-335 and reuse of M6 as a comparatorin regions 315 and 325 reduces the area requirements of gate drivercircuit 600 while allowing it to detect the regions of operation andincrease or decrease drive current Idrive accordingly. The changingdrive current Idrive reduces propagation delay as M_H1 turns on and offand the corresponding dead time without generating problematic EMI anddisrupting operation of other circuits in the IC.

As discussed previously, gate driver circuit 600 receives two inputsfrom charge pump protection circuit 460. One input is supplied to thesource terminal of M1 during region 310 and increases current suppliedby current mirror 430. Another input is supplied to the gate terminal ofM13 during turn off and biases M13 on and acting as a closed switch.FIG. 7 illustrates Vgs_HS 360, the impedance 710 seen at the sourceterminal of M1 from the charge pump protection circuit 460, the current720 input to and output by current mirror 430, and the mono-shot signal730 from charge pump protection circuit 460 applied to the gate terminalof M13 over time.

During regions 305 and 315, the impedance 710 from charge pumpprotection circuit 460 is greater than during region 310. The decreasedimpedance 710 in region 310 increases the current 720 input to andoutput by current mirror 430. This increased current 720 is solelyresponsible for the drive current supplied to output node 495 duringregion 310 and is chosen such that the slew rate complies with EMIrequirements of the IC. Increasing and maintaining the increasedimpedance 710 before and after region 310 reduces the current drawn fromthe charge pump. As explained previously, in the event of a shortcircuit on node 115, M6 and M8 automatically disconnect the charge pumpfrom output node 495 while the increased impedance from charge pumpprotection circuit 460 carefully controls the current 720 drawn from thecharge pump by current mirror 430, preventing charge pump collapse.

In some charge pump protection circuits, the mono-shot signal 730 isapplied to the gate terminals of one or more transistors within thecharge pump protection circuit. During region 310, mono-shot signal 730is logic high and causes the one or more transistors within the chargepump protection circuit to be on and to act as closed switches,decreasing the impedance 710 from the charge pump protection circuit460. While this also causes M13 to act as a closed switch, M13 isdisconnected from output node 495 because M_Inv2 is off and acts as anopen switch. During regions 325-335, mono-shot signal 730 is logic high,biasing M13 on and acting as a closed switch. M_Inv2 is also on andacting as a closed switch. This creates a path for Ipulldown current.While this also causes the impedance 710 from the charge pump protectioncircuit 460 to decrease, M1 is off and acts as an open switch, so noadditional current 720 is drawn from the charge pump.

In this description, the term “couple” or “couples” means either anindirect or direct wired or wireless connection. Thus, if a first devicecouples to a second device, that connection may be through a directconnection or through an indirect connection via other devices andconnections. The recitation “based on” means “based at least in parton.” Therefore, if X is based on Y, X may be a function of Y and anynumber of other factors.

Modifications are possible in the described embodiments, and otherembodiments are possible, within the scope of the claims.

What is claimed is:
 1. A pull-down driver circuit, comprising: a firstsub-circuit configured to generate a drive current output by thepull-down driver circuit through an output node during a first region ofoperation and a second region of operation, comprising: a diode coupledto the output node and a first transistor, and a second transistorcoupled to the first transistor and a current mirror; a secondsub-circuit configured to generate the drive current during the firstand the second regions of operation and a third region of operation,comprising: a third transistor coupled to the output node; and a fourthtransistor; and a third sub-circuit configured to generate the drivecurrent during the third region of operation, comprising: a currentsource coupled to the current mirror and a buffer; and a fifthtransistor coupled to the third transistor and the fourth transistor andconfigured to receive an output of the buffer.
 2. The pull-down drivercircuit of claim 1, wherein the first sub-circuit is configured toreduce the drive current generated in response to a voltage on theoutput node being within a particular range from a supply voltage plus avoltage across the diode plus a threshold voltage of the firsttransistor.
 3. The pull-down driver circuit of claim 1, wherein thefirst sub-circuit is configured to stop generating drive current inresponse to a voltage on the output node equaling a supply voltage plusa voltage across the diode plus a threshold voltage of the firsttransistor.
 4. The pull-down driver circuit of claim 3, wherein thefirst transistor is configured to receive the supply voltage and furtherconfigured to turn off in response to a voltage on the output nodeequaling the supply voltage plus a voltage across the diode plus athreshold voltage of the first transistor.
 5. The pull-down drivercircuit of claim 1, wherein the current mirror is configured to stopgenerating current in response to the first sub-circuit generating nodrive current.
 6. The pull-down driver circuit of claim 5, wherein thefifth transistor is configured to turn on in response to the currentmirror generating no current.